Looking for tips on making 2.5 x 1.25 mtr table

Hi,

I'm trying to find suitable materials to make a frame approximately 2500x1250mm to support a light gantry (5kg Inc router).

Originally I had planned to fabricate the frame from 50x50x3 rhs but I'm rethinking using aluminium profile, maybe 40x40 t slot or something, not sure how rigid it would be though.

Does anyone have any ideas or experience in building a table so big please? I have found UK suppliers of the extrusion but if it needs lots of bracing I fear it will cost too much :(

Any tips or links would be much appreciated.

Thanks very much,

JJ

Go with the 50x50RHS IMO. It will be so much stiffer you would not believe, presuming the joints are welded of course. Just make sure the RHS you buy is as straight as possible. I'd like to see a design for a 5kg gantry spanning 1250mm, think you might have to defy the physical laws of the universe to achieve it but I'm always eager to be educated :)

Don't be too afraid of weight in either your table or gantry, the mass will absorb a lot of the harmonics generated by the router and you wil get much smoother and accurate cuts.

Just my opinion, probably get shot down now :)

Jeff.

Thanks Jeff, I think welded rhs is the way too. The design I'm constructing is here http://automatedwoodworks.com/All%20axises_640x480.jpg what do you think of the gantry? It's made using 4x2 aluminium channel.

What router are you planning to use? The 4x2 aluminium channel (assuming 3.2mm wall thickness/10SWG) weighs in at just on 3kg, and the z-plate is around 2.4kg (based on approx measurements taken off pic) so your gantry, including slides etc, is going to weigh in at 6kg+ plus a router, which typically weigh in at 2 - 3kg if you remove the extraneous gubbins. A typical gantry inc router & cutter would be ~10kg.

But as Jeff says, thats not too big an issue, mass helps (except in acceleration of course). What sort of drive mechanism were you planning for your X-axis?

Welded steel is rigid if suitably braced but care is needed to avoid warping due to heating effects...

the machine is very similar to mine except they are driving it from under or at the side of the table using belts. Not sure about the gantry, I think it will twist easily especially if you decide to hang a heavier router or spindle on it later (and you will, believe me). Have a look at my build, it would be easy to adapt the gantry design to this table and it would be so much stiffer. You don't have to go to ballscrews, it would be just as easy to use belts if you want to keep the cost down. I've never understood why they need to drive the gantry from so low down. Mine is driven by rack and pinion from above the table by a single stepper driving both sides just below the supported rails holding the gantry. It works really well and because the drive is almost in the same plane as the supported rail, there is much less strain on the drive train/bearings.

Jeff.

Thanks guys, I'm using the gecko 540 with nema 23 sm from cncrouterparts. I will look at your build Jeff as I may revise the gantry slightly. I'm not too concerned about possible gantry twist as the gantry is driven from both sides, I personally think this will eliminate any possible problems. You should look at the designers video and work produced with this machine, it's highly impressive with excellent travel speeds and tolerances. I'll post some pics as the stuff arrives.

Thanks again for your time, the advice is invaluable as it's my first build :)

Hi Psiron,

I didn't mean twist in that way, that would be racking where one side moves more than the other. What I mean is physically twisting the 4x2 channel which will show up as little ridge lines on your cuts. When I first put my design up, Irving and a couple of others commented that as the rails were unsupported the gantry would twist and they were 100% correct, I soon changed to profiled rail mounted on the steel box section and all my problems went away. I think you'd see a similar situation midway across the table. I suppose a lot depends on what you want to produce witht he machine, as it's an 8 x 4 I presume you are looking at cutting sheet stock so this might not be much of a concern if it's just shapes you are cutting.

I'll have a look at the videos (didn't see them) and compare them with how my machine performs

Jeff.

Quote:

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Originally Posted by irving2008

Welded steel is rigid if suitably braced but care is needed to avoid warping due to heating effects...

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I always forget that unless you are "in the game" this might be an issue :)

Its always amazed me just how flat you can get a piece of box section with a little heat shrinking. Good point though.

Jeff.

Psiron,

Looked at the video and a couple of things stand out.

He only shows the machine doing rapids and he has set very low accellerations on the two main axes. This sort of negates his claim of 1000 IPM, it will average considerably less. As he states himself it only gets to that speed just before it hits the limits. Don't be lured into the rapids trap. Most of the cutting you do will be at or near feedrate speeds and I can't find a video of it cutting the pieces in the gallery. I still think that gantry will twist under cutting forces. Of course you can still get the sort of finish he has got in the gallery from a bendy machine, you just need to slow down the feedrate and reduce the cut/stepover/stepdown etc. It's a PITA though and much better to do it right first time.
BTW, speaking of limits, you really shouldn't use a limit switch as a physical stop as he seems to have done. On my machine the limit switch is set to trigger at least 25mm before the gantry hits a stop, the limit is a roller type switch and the trigger is a wedge of plastic. My gantry will actually roll off the end of the rack before it hits a stop so no mechanical damage can occur.

Someone else (Irving?) might be better able to comment on the long term viability of using belt drives over such large distances. 16 feet of belting per side is surely going to have some sort of backlash in it. I know small belts are regarded as "zero backlash" but at these lengths, ambient heat will surely have some effect plus the belt must sag as it expands with working. It might be no more than microns but it might be much more.I'm stabbing in the dark here but these seem like common sense questions to ask. I do like that he is driving both sides of the long axis from a single motor, you are not going to get racking that way but what if one belt expands a little more than the other?

It look a good sensible design overall and one that can be easily modified if things don't work out as you wanted.

Jeff.

Hey Jeff,

Ill be the first to admit I know very little about the CNC DIY scene. The reason I chose to use this design is I personally like the simplicity of the design and the fact that I can source many of the parts for little expense.

I have asked questions regarding the backlash and have also read his build log on cnczone. I see from the build log he has asked lots of questions and made changes when he has been able to improve the design of the machine. Based on contact with the designer and viewing threads he has started I am satisfied that I could be following a worse design.

I must admit I too was curious about the wear of the belt and did ask the question about the backlash, he has indicated that there is none after months of use however he did admit this is something that can vary depending on the use of the machine. I did call fenners in the UK who confirmed that the belts I will be using are designed for this purpose and should perform to a high standard. I am realistic enough however to accept that the system may need revising at some stage as you have pointed out.

I see many people going on about rapids and tbh it doesnt bother me, personally I think anyone cutting at these speeds would be mad. I am happy to walk away from the machine letting it jog along all day. After all even at low speeds it would out perform me all day long.

One thing I do not like however is the limits as you pointed out. I will certainly be installing a 'hard stop' as the last thing I need is the gantry falling off!!

Furthermore with regards to the gantry twist I do feel I would like to do my best to eliminate any possibility of this. Would you have any advice on how to possibly overcome this? I did try to look at your gantry but the photos are not shwoing?

I appreciate your input in this, the community here is awesome. Once I have received the tables from the fabricator I will start a build log and post some photos.

Thanks again for your reply, hopefully I can resolve some of these problems before they start!

:)